The present invention relates to novel materials with electrochemical properties, in particular paste-like masses, layers produced from these paste-like masses that are self-supporting or that are placed on a substrate, and composite layers produced therefrom that can be used as accumulators, electrochromic elements, or the like. The invention particularly relates to rechargeable electrochemical cells on a fixed body base.
Since the beginning of the 1970""s there have been attempts to produce electrochemical elements such as accumulators or the like in the form of thin layers. The goal has been to obtain composite films that are both flexible enough that they can be, for instance, rolled up or made to conform to another desired shape and that also have particularly good charging and discharging properties due to an extremely high contact area between the individual electrochemical components, such as electrodes and electrolytes, relative to the volume of active electrochemical material used.
In the past, attempts to produce such electrode materials have begun with solid or viscous liquid Teflon, which is mixed with a certain percentage of carbon and the actual electrode material and is then pressed or sprayed onto suitable reference electrodes. However, this results in layers that have insufficient flexibility. In addition, it has been suggested that electrode layers be produced that are manufactured with PVC and tetrahydrofurane or another polymer dissolved in a solvent and that the solvent subsequently be extracted therefrom. However, the conductivity of products produced in this manner is not favorable.
Producing a layer that can function in an appropriate electrochemical composite as an electrolyte presents particular problems. U.S. Pat. No. 5,456,000 describes rechargeable battery cells that are produced by laminating electrode and electrolyte cells. Used for the positive electrode is a film or membrane that is produced separately from LiMn2O4 powder in a matrix solution made of a copolymer and is then dried. The negative electrode comprises a dried coating of a pulverized carbon dispersion in a matrix solution of a copolymer. An electrolyte/separator membrane is arranged between the electrode layers. For this purpose a poly(vinylidene fluoride)-hexafluoropropylene copolymer is converted with an organic plasticizer such as propylene carbonate or ethylene carbonate. A film is produced from these components and then the plasticizer is extracted from the layer. The battery cell is maintained in this xe2x80x9cinactivexe2x80x9d condition until it is to be used. In order to activate it, it is immersed in a suitable electrolyte solution, whereby the cavities formed by extracting the plasticizer are filled with the liquid electrolytes. The battery is then ready for use.
Such a construct is disadvantageous in that the battery cannot be maintained for extended periods in a charged condition because corrosion occurs at the limit surfaces (see oral presentation made by A. Blyr et. al., 4th Euroconference on Solid State Ionics, Connemara, Ireland, September 1997, provided for publication). The use of a liquid electrolyte thus entails stability problems at the phase limits in the composite layer. Another disadvantage is that the battery must be arranged in a housing that is leak-proof.
There have already been attempts to use solid electrolytes. It has been suggested that ion-conducting organic polymer materials be used (so-called true polymer electrolytes). Thus, U.S. Pat. No. 5,009,970 describes using a gel product that is obtained by converting a solid poly(ethylene oxide) polymer with lithium perchlorate and then irradiating it. U.S. Pat. No. 5,041,346 describes an oxymethylene cross-linked variant of these polymer electrolytes that also contains a softener that preferably has ion-solvating properties, for example, that can be a dipolar aprotic solvent such as g-butyrolactone. However, it has been reported that although the ion conductivity compared to pure solid lithium is drastically elevated, it is still not sufficient for use as an electrolyte layer in electrochemical elements.
Another attempt to use solid electrolytes has involved similar polymer electrolytes. In this case polyvinylfluoride polymers and related fluorocarbon copolymers were used with trifluoroethylene or tetrafluoroethylene. Added to these polymers were lithium salts and additional organic solvents that were compatible both with the polymers and with the salt components (Tsuchida et. al., Elektrochimica Acta, Volume 28 (1983, page 591 ff and page 833 ff). However, in this case a usable ion conductivity of greater than about 10xe2x88x925S/cm can only be obtained at elevated temperatures because, as the authors themselves reported, this mixture did not remain homogeneous; rather, it formed salt and polymer crystallite. Research in this direction was therefore later deemed unpromising (see U.S. Pat. No. 5,456,000, column 2, lines 31 through 33).
The object of the present invention is to provide masses for producing electrochemical elements in the form of thin composite layers that do not have the aforesaid unfavorable properties. In particular the inventive masses, when processed into layers or composite layers with electrochemical properties, should provide products such as rechargeable batteries (accumulators), electrochromic elements, or the like, that have a high degree of flexibility and very good electron- and ion-conducting properties and that furthermore cannot leak and therefore do not have to be maintained in housings, especially in sealing housings.
This object is achieved in that, in accordance with the invention, paste-like masses that can be used in electronic elements are prepared that include a heterogeneous mixture of (A) a matrix containing or comprising at least one organic polymer, precursors thereof, or prepolymers thereof, and (B) an inorganic material that can be electrochemically activated, is not soluble in the matrix, and is in the form of a solid substance. The mixture contains no lithiated zeolites, nor any electron or ion conducting organic polymers.
The term xe2x80x9cthat can be used in electrochemical elementsxe2x80x9d implies that the electrochemically activatable inorganic material that is in the form of a solid substance must be an ion-conducting or electron-conducting material that is suitable for electrode material or for a solid electrolyte.
In accordance with the invention at least one additional condition must be satisfied so that there is sufficient electrical contact between the individual grains of the electrochemically activatable solid substance (B) that is embedded in the matrix (A). Namely, it has been demonstrated that the poor conductivity described in the prior art cannot be overcome unless the mass contains a sufficient quantity of electrochemically activatable solid substance. Very good conductivity, or even sufficient conductivity, cannot be achieved unless the proportional volume of the electrochemically activatable solid substance is so high that it is approximately equal to the filled space in a theoretical close-pack. The minimum can vary somewhat depending on the materials used, since naturally parameters such as size and external shape of the electrochemically activatable solid substance (B) obviously play a role. However, it is recommended that at least 60 volume % of solid substance (B) be used, preferably a minimum of about 65 volume %, and particularly preferably a minimum of about 70 volume %. The upper limit is not critical; it depends primarily on the properties of the matrix (A). If it [the matrix] has very good adhesion, it is possible to work into the paste-like mass up to 90 volume %, in exceptional cases even up to 95 volume %, of solid substance (B).
However, alternatively or in addition, it is also possible to achieve sufficient electrical contact between the grains of the solid substance (B) in that a second ion- and/or electron-conductor (or a homogeneous, mixed conductor, depending on the type of conductivity needed) (C) is used that is present as a thin layer, at least at the grain limits between (A) and (B).
Additional conditions variant (a) or (b), must be met if the solid substance (B) is an electrode material.